Abnormal Changes Diagnosis and its ControlFactors of Water and Sediment Dischargein the Lower Reaches of the Three LargestRivers in China from 1960 to 2020

There are general trends and significant and abnormal changes in water and sediment discharge in rivers and estuaries, and the abnormal changes have been diagnosed frequently.Here we clarified the time node and mechanism of abnormal changes in water and sediment discharges in the lower reaches of the three largest rivers in China, which occurred with the increase of strong human activities and climate change.Using the Mann-Kendall test, Pettitt test, and double cumulative curve methods, we analyzed the data change characteristics of 5 hydrological stations: Lijin (Yellow River), Datong (Yangtze River), Gaoyao, Shijiao, and Boluo (Pearl River) from 1960 to 2020.The results showed that the Yellow River had significant and abnormal changes in water discharge in 1976 and 1986 separately, while the Yangtze River and Pearl River didn't show abnormal changes due to being located in different climatic zones.In addition, the Yellow and Yangtze Rivers had abnormal changes in sediment discharge in 1997 and 1992, separately, the West and East branch rivers of the Pearl River showed abnormal changes in 2000 and 1989, separately, and the North branch rivers of the Pearl River only showed significant changes in 1999.Most sediment abnormal changes for the three rivers occurred in 10 years, from 1990-2000, when there were strong human activities, including water and soil conservation, sand mining, and dam construction.Dam construction plays the most important role in sediment abnormal changes.The intensity after the sediment abnormal change is around 5 times that before the abnormal change in the Yellow River, but the intensity is only 2-3 times in the Yangtze River and Pearl River.The Yellow River shows a higher vulnerability to water and sediment changes than the other two rivers under the influence of strong human activities and thus needs to be carefully protected for sustainable use.